1. Field of the Invention
This invention relates generally to a receiver, and more particularly to a receiver having improved automatic gain control characteristics.
2. Description of the Prior Art
In the prior art, a receiver for amplitude modulated signals usually includes an automatic gain control system which keeps the output signal of the receiver relatively constant despite variations in signal strength at the antenna of the receiver. The automatic gain control system makes it possible to tune from a weak signal to a strong signal without having the audio output become inordinately loud. In other words, it is important for a receiver to automatically vary the gain of the receiver in such a manner that the receiver will receive a weak signal with high sensitivity but a strong signal with low sensitivity.
In general, an automatic gain control system used in a prior art receiver carries out its automatic gain control operation by rectifying an audio signal and controlling the gain of, for example, an RF (radio frequency) signal amplifier or IF (intermediate frequency) signal amplifier with the rectified audio signal. In a transistorized receiver, the above rectified output signal is supplied to the base bias circuit for the transistor which forms an amplifier to vary the base bias of the transistor and hence to control the gain of the amplifier.
In an IF stage in which a pair of transistors form a differential amplifier, a gain control signal is applied to the base electrodes of the pair of transistors or to the base electrode of a transistor which forms a constant current circuit. In many cases, however, an amplifier whose gain is to be controlled has connected, at its input side, a resistor which forms a voltage dividing circuit together with a variable impedance element whose impedance varies when a gain control signal is applied to the input side of the amplifier. The resistor is desired to be omitted when automatic gain control operation is not performed, because the resistor causes undesired power consumption and signal loss.